Aluminum plating film and method for producing aluminum plating film

ABSTRACT

An aluminum plating film contains aluminum as a main component. The aluminum plating film has, between coating surfaces at both ends in a thickness direction, an intervening layer that contains a metal having a lower ionization tendency than aluminum or an intervening layer that contains an alloy of aluminum and a metal having a lower ionization tendency than aluminum.

TECHNICAL FIELD

The present disclosure relates to an aluminum plating film and a methodfor producing an aluminum plating film.

The present application claims priority from Japanese Patent ApplicationNo. 2017-097138 filed on May 16, 2017, and the entire contents of theJapanese patent application are incorporated herein by reference.

BACKGROUND ART

PTL 1 discloses that a porous plate formed of an insulating material isdisposed between a substrate and an anode. An opening ratio of theporous plate described in PTL 1 is adjusted along a direction in whichthe substrate travels to make a current density distribution uniform inthe direction in which the substrate travels. As a result, an aluminumplating film having a uniform thickness can be formed.

PTL 2 discloses that an aluminum plating film is formed on a substrateby using a continuous electrical treatment apparatus that includes aplating bath having a cell structure in which an upper tank and a lowertank are connected to each other with two passages therebetween, a sinkroll disposed in the lower tank, and a power supply roll disposedoutside the plating bath and above the upper tank. According to thecontinuous electrical treatment apparatus described in PTL 2, analuminum plating film can be formed on a surface of a substrate aplurality of times by reciprocating the substrate between the powersupply roll and the sink roll a plurality of times.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 10-317195

PTL 2: Japanese Unexamined Patent Application Publication No. 11-117089

SUMMARY OF INVENTION

An aluminum plating film of the present disclosure is an aluminumplating film containing aluminum as a main component. The aluminumplating film has, between coating surfaces at both ends in a thicknessdirection, an intervening layer that contains a metal having a lowerionization tendency than aluminum or an intervening layer that containsan alloy of aluminum and a metal having a lower ionization tendency thanaluminum.

A method for producing an aluminum plating film of the presentdisclosure is a method for producing the aluminum plating film of thepresent disclosure, the method including a first electrolytic treatmentstep of forming a pre-aluminum plating film by subjecting a substrate,at least a surface of the substrate being conductive, to an electrolytictreatment in a first electrolyte to electrodeposit aluminum on a surfaceof the substrate; a replacement step of, after the first electrolytictreatment step, forming a replacement plating film by immersing thepre-aluminum plating film in a replacement solution that contains thefirst electrolyte and a metal having a lower ionization tendency thanaluminum to replace a surface of the pre-aluminum plating film with themetal having a lower ionization tendency than aluminum; and a secondelectrolytic treatment step of, after the replacement step, forming analuminum plating film by subjecting the replacement plating film to anelectrolytic treatment in a second electrolyte to electrodepositaluminum on a surface of the replacement plating film. The firstelectrolyte and the second electrolyte are each a molten salt thatcontains at least aluminum chloride.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an enlarged view that schematically illustrates a section ofan example of an aluminum plating film according to an embodiment of thepresent disclosure.

FIG. 2 is an enlarged view that schematically illustrates a section ofanother example of an aluminum plating film according to an embodimentof the present disclosure.

FIG. 3 is an enlarged view that schematically illustrates a partialsection of still another example of an aluminum plating film accordingto an embodiment of the present disclosure.

FIG. 4 is an enlarged view that schematically illustrates a sectiontaken along line A-A in FIG. 3.

FIG. 5 is a photograph of a urethane foam resin, which is an example ofa resin molded body having a skeleton with a three-dimensional networkstructure.

FIG. 6 is an enlarged view that schematically illustrates a partialsection of an example of a state in which a conductive layer is formedon a surface of a skeleton of a resin molded body having the skeletonwith a three-dimensional network structure.

FIG. 7 is a photograph showing the result of scanning electronmicroscopic observation of a section of an aluminum plating film No. 3prepared in Example 3.

FIG. 8 is a photograph showing the result of scanning electronmicroscopic observation of a section of an aluminum plating film No. 4prepared in Example 4.

FIG. 9 is a photograph showing the result of scanning electronmicroscopic observation of a section of an aluminum plating film No. 7prepared in Comparative Example 2.

DESCRIPTION OF EMBODIMENTS Problems to be Solved by Present Disclosure

Molten salt baths are used for performing electroplating of aluminum.However, since the molten salt baths have low electrical conductivities,an aluminum plating film cannot be formed at high speed. In particular,in the case of an elongated substrate, such as a steel strip, analuminum plating film having a sufficient thickness cannot be formed ina plating bath unless a transport speed is set to low. According to themethod described in PTL 1, the thickness of an aluminum plating film canbe made uniform. However, in the case where the thickness of thealuminum plating film is increased, there is no choice but to decreasethe transport speed, resulting in a decrease in production efficiency.

When the substrate has a sheet-like shape such as a steel strip, acontinuous electrical treatment apparatus as described in PTL 2 cannotbe used. In the continuous electrical treatment apparatus described inPTL 2, a substrate can be reciprocated between the power supply roll andthe sink roll a plurality of times because the substrate is a wire rod.In the case of a sheet-like substrate, the substrate cannot bereciprocated between the power supply roll and the sink roll a pluralityof times. Furthermore, aluminum is not electrodeposited on the surfaceof a part of the substrate, the part being in contact with the sink rollin the plating bath, and thus the resulting aluminum plating film has anuneven thickness.

In view of this, the inventors of the present invention have examinedthat, in order to form an aluminum plating film having a large thicknesswith high efficiency, an aluminum plating film is formed in multiplestages using a plurality of plating devices configured to form analuminum plating film while transporting a substrate in a horizontaldirection, thereby increasing the transport speed. However, thefollowing has been found. Since aluminum is a metal that is verysusceptible to oxidation, an oxide film is formed when the substrate ispulled up from a molten salt bath. Accordingly, when aluminum plating isperformed in multiple stages, such an oxide film is inserted, like anannual growth ring, between aluminum plating films formed in therespective plating devices. Even when molten salt aluminum plating isperformed in a nitrogen atmosphere (N₂: 99.99% or more) or in an argonatmosphere (Ar: 99.99% or more), an oxide film is formed on the surfaceof an aluminum plating film by a very small amount of oxygen containedin the atmosphere.

It has also been found that since the oxide film formed on the surfaceof the aluminum plating film is very dense and strong and a currentcannot be uniformly allowed to flow, the adhesion between the aluminumplating films formed in multiple stages is poor, which may result indefects such as blistering.

Accordingly, an object of the present disclosure is to provide analuminum plating film having a large thickness and capable of beingproduced within a short time at a low cost and a method for producingthe aluminum plating film.

Advantageous Effects of Present Disclosure

According to the present disclosure, it is possible to provide analuminum plating film having a large thickness and capable of beingproduced within a short time at a low cost and a method for producingthe aluminum plating film.

Description of Embodiments of Present Disclosure

First, embodiments of the present disclosure will be listed anddescribed.

(1) An aluminum plating film according to an embodiment of the presentdisclosure is an aluminum plating film containing aluminum as a maincomponent. The aluminum plating film has, between coating surfaces atboth ends in a thickness direction, an intervening layer that contains ametal having a lower ionization tendency than aluminum or an interveninglayer that contains an alloy of aluminum and a metal having a lowerionization tendency than aluminum.

According to the embodiment of the invention according to (1) above, itis possible to provide an aluminum plating film having a large thicknessand capable of being produced within a short time at a low cost.

(2) In the aluminum plating film according to (1) above, the aluminumplating film preferably has an elongated sheet-like shape. According tothe embodiment of the invention according to (2) above, it is possibleto provide an aluminum plating film that has a large thickness and thatis capable of being produced on a surface of a substrate having anelongated sheet-like shape within a short time at a low cost.(3) In the aluminum plating film according to (1) above, the aluminumplating film preferably forms a skeleton of a metal porous body, theskeleton having a three-dimensional network structure.

According to the embodiment of the invention according to (3) above, itis possible to provide an aluminum plating film that forms a skeleton ofa metal porous body, the skeleton having a large thickness, and that iscapable of being produced within a short time at a low cost.

(4) In the aluminum plating film according to (3) above, the metalporous body preferably has an elongated sheet-like shape.

According to the embodiment of the invention according to (4) above, itis possible to provide an aluminum plating film that forms a skeleton ofa metal porous body having an elongated sheet-like shape as a whole, theskeleton having a large thickness, and that is capable of being producedwithin a short time at a low cost.

(5) In the aluminum plating film according to any one of (1) to (4)above, the aluminum plating film preferably has a plurality of theintervening layers in the thickness direction of the aluminum platingfilm.

According to the embodiment of the invention according to (5) above, itis possible to provide an aluminum plating film having a largerthickness and capable of being produced within a shorter time at a lowcost.

(6) In the aluminum plating film according to any one of (1) to (5)above, the metal having a lower ionization tendency than aluminum ispreferably at least one selected from the group consisting of iron (Fe),zinc (Zn), zirconium (Zr), manganese (Mn), nickel (Ni), and copper (Cu).

According to the embodiment of the invention according to (6) above, itis possible to provide an aluminum plating film having a sufficientlyhigh conductivity.

(7) In the aluminum plating film according to any one of (1) to (6)above, the aluminum plating film preferably has a thickness of 10 μm ormore and 1,000 μm or less.

According to the embodiment of the invention according to (7) above, itis possible to provide an aluminum plating film having a largerthickness and capable of being produced within a short time at a lowcost.

(8) A method for producing an aluminum plating film according to anembodiment of the present disclosure is a method for producing thealuminum plating film according to (1) above, the method including afirst electrolytic treatment step of forming a pre-aluminum plating filmby subjecting a substrate, at least a surface of the substrate beingconductive, to an electrolytic treatment in a first electrolyte toelectrodeposit aluminum on a surface of the substrate; a replacementstep of, after the first electrolytic treatment step, forming areplacement plating film by immersing the pre-aluminum plating film in areplacement solution that contains the first electrolyte and a metalhaving a lower ionization tendency than aluminum to replace a surface ofthe pre-aluminum plating film with the metal having a lower ionizationtendency than aluminum; and a second electrolytic treatment step of,after the replacement step, forming an aluminum plating film bysubjecting the replacement plating film to an electrolytic treatment ina second electrolyte to electrodeposit aluminum on a surface of thereplacement plating film. The first electrolyte and the secondelectrolyte are each a molten salt that contains at least aluminumchloride.

According to the embodiment of the invention according to (8) above, itis possible to provide a method for producing an aluminum plating film,the method being capable of producing an aluminum plating film having alarge thickness within a short time at a low cost.

(9) In the method for producing the aluminum plating film according to(8) above, the substrate is preferably a resin molded body having askeleton with a three-dimensional network structure.

According to the embodiment of the invention according to (9) above, itis possible to provide an aluminum plating film that has a largethickness and that is capable of being produced on a surface of asubstrate having a skeleton with a three-dimensional network structurewithin a short time at a low cost.

(10) The method for producing the aluminum plating film according to (8)or (9) above preferably includes a removal step of removing thesubstrate after the second electrolytic treatment step.

According to the embodiment of the invention according to (10) above, itis possible to provide a method for producing an aluminum plating film,the method being capable of producing the aluminum plating filmaccording to (3) above.

Details of Embodiments of Present Disclosure

Specific examples of an aluminum plating film and a method for producingthe aluminum plating film according to embodiments of the presentdisclosure will be described below. The present disclosure is notlimited to these examples, but is defined by the appended claims, and isintended to cover all modifications within the meaning and scopeequivalent to those of the claims.

Aluminum Plating Film

An aluminum plating film according to an embodiment of the presentdisclosure contains aluminum as a main component and has an interveninglayer between coating surfaces at both ends in a thickness direction.The phrase “contains aluminum as a main component” as used herein meansthat the aluminum content in an aluminum plating film is 50% by mass ormore.

FIG. 1 is an enlarged schematic view of a section of an example of analuminum plating film according to an embodiment of the presentdisclosure. As illustrated in FIG. 1, an aluminum plating film 10 has anintervening layer 12 between coating surfaces 11 and 11′ at both ends inthe thickness direction. The aluminum plating film 10 may be formed on asurface of a substrate that is not illustrated. When the aluminumplating film 10 is formed on a surface of a substrate, one coatingsurface of the coating surfaces 11 and 11′ is in close contact with thesurface of the substrate. When the substrate is an elongated sheet-likesubstrate, the aluminum plating film 10 formed on the surface of thesubstrate also has an elongated sheet-like shape.

The intervening layer 12 is a layer that contains a metal having a lowerionization tendency than aluminum or a layer that contains an alloy ofaluminum and a metal having a lower ionization tendency than aluminum.The intervening layer 12 is formed on a surface substantially parallelto the coating surfaces 11 and 11′ of the aluminum plating film. Whenthe aluminum plating film 10 is an aluminum plating film formed on asurface of a flat-plate substrate, the intervening layer 12 issubstantially parallel to the coating surfaces 11 and 11′ over thesubstantial entirety of the aluminum plating film 10. When the aluminumplating film 10 is an aluminum plating film formed on a surface of asubstrate having a complex three-dimensional shape, the interveninglayer 12 is often partially substantially parallel to the coatingsurfaces 11 and 11′ of the aluminum plating film 10.

The thickness of the intervening layer 12 is preferably as small aspossible, and the intervening layer 12 may be a one-atomic layer. Theupper limit of the intervening layer 12 is not particularly limited butis, for example, about 500 nm or less. With a decrease in the thicknessof the intervening layer 12, adhesion force between aluminum layers thatare formed on both sides of the intervening layer 12 increases, and adecrease in the conductivity of the aluminum plating film can beprevented. The thickness of the intervening layer 12 is more preferably1 nm or more and 400 nm or less, still more preferably 10 nm or more and200 nm or less.

The intervening layer included in the aluminum plating film according toan embodiment of the present disclosure can be confirmed by, forexample, observing a section of the aluminum plating film with ascanning electron microscope (SEM) or a transmission electron microscope(TEM). In the case where the thickness of the intervening layer is about50 nm or more, the intervening layer can be confirmed with a SEM. In thecase where the thickness of the intervening layer is less than about 50nm, the intervening layer can be confirmed by elemental mapping with aTEM.

FIG. 2 is an enlarged schematic view of a section of another example ofan aluminum plating film according to an embodiment of the presentdisclosure. As illustrated in FIG. 2, an aluminum plating film 20preferably has a plurality of intervening layers 22 between coatingsurfaces 21 and 21′ at both ends in the thickness direction. Since thealuminum plating film 20 has the plurality of intervening layers 22, thealuminum plating film 20 can be produced within a shorter time at a lowcost. When the aluminum plating film has a plurality of interveninglayers, the intervening layers are also substantially parallel to eachother.

The metal that constitutes the intervening layer and that has a lowerionization tendency than aluminum is preferably at least one selectedfrom the group consisting of iron (Fe), zinc (Zn), zirconium (Zr),manganese (Mn), nickel (Ni), and copper (Cu). Since these metals havesufficiently high conductivity, the metals do not decrease theconductivity of the aluminum plating film even when they are present asan intervening layer in the aluminum plating film.

When the metal having a lower ionization tendency than aluminum is Fe,Zn, Cu, or, Ni, the aluminum plating film can be produced at arelatively low cost.

When the metal having a lower ionization tendency than aluminum is Zr,adhesion between aluminum layers formed on both sides of the interveninglayer can be further enhanced.

When the metal having a lower ionization tendency than aluminum is Mn,the aluminum plating film can also be suitably used as a currentcollector of a nonaqueous electrolyte battery or the like because Mn hashigh corrosion resistance.

The metal having a lower ionization tendency than aluminum may form analloy with aluminum in the intervening layer.

When the aluminum plating film is formed on a surface of a substrate, itis preferable that the thickness of the aluminum plating film be aslarge as possible from the viewpoint of protecting the substrate. Thethickness of the aluminum plating film is preferably about 10 μm ormore. When the thickness of the aluminum plating film is 10 μm or more,rupture strength of the aluminum plating film can be increased. Thethickness of the aluminum plating film is preferably about 1,000 μm orless from the viewpoint of the production cost and the reduction in theweight. From these viewpoints, the thickness of the aluminum platingfilm is more preferably 15 μm or more and 700 μm or less, still morepreferably 20 μm or more and 500 μm or less.

In the related art, it has taken a lot of time to produce an aluminumplating film having a thickness of 10 μm or more. Furthermore, it hasbeen very difficult to form an aluminum plating film that has anelongated sheet-like shape and that has a thickness of 10 μm or more.

In contrast, according to the aluminum plating film according to anembodiment of the present disclosure, an aluminum plating film can beproduced within a short time at a low cost even when the aluminumplating film has a thickness of 10 μm or more, and an aluminum platingfilm that has an elongated sheet-like shape can also be easily produced.Furthermore, the aluminum plating film according to an embodiment of thepresent disclosure has no gap in the film and is dense. Accordingly,even when the thickness of the aluminum plating film is increased, theconductivity does not decrease, and the aluminum plating film has aconductivity substantially equal to that of a rolled foil.

As illustrated in FIG. 3, the aluminum plating film according to anembodiment of the present disclosure may form a skeleton 33 of a metalporous body, the skeleton 33 having a three-dimensional networkstructure. In the example illustrated in FIG. 3, an inside 34 of theskeleton 33 is hollow. Alternatively, a substrate may be present in theinside 34 of the skeleton 33. The metal porous body has a continuouspore, and a pore 35 is formed by the skeleton 33.

FIG. 4 is an enlarged view that schematically illustrates a sectiontaken along line A-A in FIG. 3. As illustrated in FIG. 4, an aluminumplating film 30 that forms the skeleton 33 of the metal porous body hasan intervening layer 32 between coating surfaces 31 and 31′ at both endportions in the thickness direction. Ideally, the intervening layer 32is substantially parallel to the coating surfaces 31 and 31′ asillustrated in FIG. 4. However, in the aluminum plating film that formsa skeleton having a complex shape as in the metal porous body having askeleton with a three-dimensional network structure, the interveninglayer 32 is in a state of being partially substantially parallel to thecoating surfaces 31 and 31′.

Even when the metal porous body has an elongated sheet-like shape as awhole, an aluminum plating film can be produced within a short time at alow cost because the skeleton is formed by the aluminum plating filmaccording to an embodiment of the present disclosure.

The porosity, the average pore diameter, and the thickness of the metalporous body are appropriately selected in accordance with theapplication of the metal porous body. For example, when the metal porousbody is used as an electrode (current collector) of a battery, a metalporous body having a small average pore diameter and a small thicknessis preferred. When the metal porous body is used for heat dissipation, ametal porous body having a large average pore diameter and a largethickness is preferred.

The porosity of the metal porous body refers to a ratio of the volume ofan internal space (pore) of the metal porous body to the apparentvolume.

The average pore diameter of the metal porous body refers to thereciprocal of the number of cells (cells/inch) formed by the skeleton ofthe metal porous body.

Method for Procuring Aluminum Plating Film

A method for producing an aluminum plating film according to anembodiment of the present disclosure is a method for producing thealuminum plating film according to an embodiment of the presentdisclosure and includes a first electrolytic treatment step, areplacement step, and a second electrolytic treatment step. In the casewhere an aluminum plating film having a larger thickness is produced, aset of a replacement step and an electrolytic treatment step may befurther repeatedly performed after the second electrolytic treatmentstep. As a result, an aluminum plating film having a plurality ofintervening layers in the aluminum plating film can be produced.Furthermore, in the method for producing an aluminum plating filmaccording to an embodiment of the present disclosure, a removal step ofremoving a substrate may be performed as required.

The respective steps will now be described in detail.

First Electrolytic Treatment Step

The first electrolytic treatment step is a step of forming apre-aluminum plating film by subjecting a substrate to an electrolytictreatment in an electrolyte to electrodeposit aluminum on a surface ofthe substrate.

The electrolytic treatment (molten salt electrolysis) can be performedby arranging a substrate and aluminum so as to face each other in anelectrolyte, connecting the substrate to the cathode side of arectifier, connecting the aluminum to the anode side, and applying avoltage between both electrodes. The first electrolytic treatment stepis performed in an inert atmosphere such as an argon gas atmosphere or anitrogen gas atmosphere.

Substrate

The substrate is not particularly limited as long as the substrate needsto have an aluminum plating film thereon. For example, a copper plate, asteel strip, a copper wire, a steel wire, or a resin molded body thathas been subjected to an electrical conduction treatment can be used asthe substrate. In the related art, it has been very difficult to form analuminum plating film having a large thickness on an elongatedsheet-like substrate, and a lot of time and a high cost have beenrequired. In contrast, according to the method for producing an aluminumplating film according to an embodiment of the present disclosure, analuminum plating film having a large thickness can be produced within ashort time at a low cost even when a substrate has an elongatedsheet-like shape.

For example, a polyurethane, melamine resin, polypropylene,polyethylene, or the like having a surface that has been subjected to anelectrical conduction treatment can be used as the resin molded bodythat has been subjected to an electrical conduction treatment. The resinmolded body that has been subjected to an electrical conductiontreatment may have any shape. However, by using a resin molded bodyhaving a skeleton with a three-dimensional network structure, a metalporous body that exhibits good properties for applications to a filter,a catalyst support, a battery electrode, and the like can be finallyproduced. As the resin molded body having a skeleton with athree-dimensional network structure, a resin foamed body is preferablyused. The resin foamed body may be any porous resin foamed body. A knownor commercially available resin foamed body can be used. For example, aurethane foam or a styrene foam can be used. Of these, in particular, aurethane foam is preferred from the viewpoint of high porosity. FIG. 5shows a photograph of a urethane foam resin. Similarly, by using a resinmolded body having a nonwoven fabric-like skeleton, a metal porous bodyhaving a nonwoven fabric-like skeleton and being capable of beingpreferably used in applications to a filter, a catalyst support, abattery electrode, and the like can be finally produced.

FIG. 6 shows an enlarged view that schematically illustrates a partialsection of an example of a substrate prepared by subjecting a resinmolded body having a skeleton with a three-dimensional network structureto an electrical conduction treatment. As illustrated in FIG. 6, a resinmolded body 66 having a skeleton with a three-dimensional networkstructure has a continuous pore, and a pore 65 is formed by theskeleton. Since the skeleton of the metal porous body is formed byforming an aluminum plating film on the surface of the skeleton of theresin molded body 66, the porosity, the average pore diameter, and thethickness of the metal porous body are substantially equal to theporosity, the average pore diameter, and the thickness of the resinmolded body 66. Therefore, the porosity, the average pore diameter, andthe thickness of the resin molded body 66 may be appropriately selectedin accordance with the porosity, the average pore diameter, and thethickness of the target metal porous body to be produced. The porosityand the average pore diameter of the resin molded body 66 are defined inthe same manner as in the porosity and the average pore diameter of themetal porous body.

The method for subjecting the surface of the skeleton of the resinmolded body 66 to an electrical conduction treatment is not particularlylimited as long as a conductive layer 67 can be formed on the surface ofthe skeleton of the resin molded body 66. Examples of the material thatconstitutes the conductive layer 67 include metals such as nickel,titanium, and stainless steel and carbon powders such as graphite andamorphous carbon, e.g., carbon black. Among these, in particular, carbonpowders are preferred, and carbon black is more preferred. In the casewhere the conductive layer 67 is formed by using amorphous carbon or acarbon powder other than a metal, the conductive layer 67 is alsoremoved together when the resin molded body is removed, as needed.

For example, in the case of using nickel, specific preferred examples ofthe electrical conduction treatment include a non-electrolytic platingtreatment and a sputtering treatment. In the case of using a materialsuch as a metal, e.g., titanium or stainless steel, carbon black, orgraphite, an example of the preferred method is a treatment of applying,to the surface of the skeleton of the resin molded body 66, a mixtureprepared by adding a binder to a fine powder of any of these materials.

In the non-electrolytic plating treatment using nickel, for example, theresin molded body 66 may be immersed in a known non-electrolyticnickel-plating bath such as an aqueous nickel sulfate solution thatcontains sodium hypophosphite as a reducing agent. Before immersion inthe plating bath, the resin molded body 66 may be immersed in, forexample, an activating liquid that contains a very small amount ofpalladium ions (cleaning liquid, produced by Japan Kanigen Co., Ltd.),as needed.

The sputtering treatment using nickel may be performed as follows, forexample. After the resin molded body 66 is attached to a substrateholder, a direct-current voltage is applied between the holder and atarget (nickel) while introducing an inert gas, thereby causing theresulting ionized inert gas to collide with the nickel. The resultingsputtered nickel particles are deposited on the surface of the skeletonof the resin molded body 66.

The conductive layer 67 is continuously formed so as to cover thesurface of the skeleton of the resin molded body 66. The coating weightof the conductive layer 67 is preferably, but is not necessarily, 1.0g/m² or more and 30 g/m² or less, more preferably 5.0 g/m² or more and20 g/m² or less, still more preferably 7.0 g/m² or more and 15 g/m² orless.

The coating weight of a conductive layer refers to the mass of theconductive layer per apparent unit area of a resin molded body thatincludes the conductive layer formed on the surface of the skeletonthereof.

Electrolyte

As the electrolyte, a molten salt (ionic liquid) capable ofelectrodepositing aluminum on the surface of the substrate may be used.Specifically, the electrolyte may be a molten salt that contains analuminum halide and may optionally contain an additive.

Examples of the aluminum halide include aluminum chloride (AlCl₃),aluminum bromide (AlBr₃), and aluminum iodide (AlI₃). Among these,aluminum chloride is most preferable.

For example, a chloride or fluoride molten salt can be preferably usedas the molten salt. Examples of the chloride molten salt that can beused include KCl, NaCl, CaCl₂, LiCl, RbCl, CsCl, SrCl₂, BaCl₂, MgCl₂,and eutectic salts thereof. Examples of the fluoride molten salt thatcan be used include LiF, NaF, KF, RbF, CsF, MgF₂, CaF₂, SrF₂, BaF₂, andeutectic salts thereof.

Among the above molten salts, KCl, NaCl, or CaCl₂ is preferably usedfrom the viewpoint of a low cost and ease of availability.

From the viewpoint of lowering the melting point, the molten saltpreferably contains at least one molten salt-forming compound selectedfrom the group consisting of alkylimidazolium halides, alkylpyridiniumhalides, and urea compounds. A compound that forms a molten salt atabout 110° C. or lower when mixed with an aluminum halide can besuitably used as the molten salt-forming compound.

Examples of the alkylimidazolium halides include imidazolium chlorideshaving alkyl groups (each having 1 to 5 carbon atoms) at the 1- and3-positions, imidazolium chlorides having alkyl groups (each having 1 to5 carbon atoms) at the 1-, 2-, and 3-positions, and imidazolium iodideshaving alkyl groups (each having 1 to 5 carbon atoms) at the 1- and3-positions.

Specific examples thereof include 1-ethyl-3-methylimidazolium chloride(EMIC), 1-butyl-3-methylimidazolium chloride (BMIC), and1-methyl-3-propylimidazolium chloride (MPIC). Among these,1-ethyl-3-methylimidazolium chloride (EMIC) can be most preferably used.

Examples of the alkylpyridinium halides include 1-butylpyridiniumchloride (BPC), 1-ethylpyridinium chloride (EPC), and1-butyl-3-methylpyridinium chloride (BMPC). Among these,1-butylpyridinium chloride is most preferred.

The urea compounds refer to urea and derivatives thereof, and, forexample, compounds represented by formula (1) below can be preferablyused.

In formula (1), R each represent a hydrogen atom, an alkyl group having1 to 6 carbon atoms, or a phenyl group, and R may be the same ordifferent from each other.

Among the above compounds, urea and dimethylurea can be particularlypreferably used as the urea compounds.

In the case where the above molten salt-forming compound is used, anelectrolyte suitable for electrodepositing aluminum on the surface ofthe substrate is obtained by adjusting a mixing ratio of the aluminumhalide to the molten salt-forming compound in a range of 1:1 to 3:1 interms of molar ratio.

Examples of the additive include smoothing agents capable of smoothing apre-aluminum plating film formed by electrodeposition on the surface ofthe substrate.

For example, at least one compound selected from the group consisting of1,10-phenanthroline chloride monohydrate, 1,10-phenanthrolinemonohydrate, and 1,10-phenanthroline can be preferably used as thesmoothing agent. When the electrolyte contains any of these smoothingagents, a smooth, mirror-like pre-aluminum plating film is formed.

Replacement Step

The replacement step is a step of immersing the pre-aluminum platingfilm formed on the surface of the substrate in the first electrolytictreatment step in a replacement solution to replace a surface of thepre-aluminum plating film with a metal other than aluminum. Thereplacement solution is a solution that contains a metal having a lowerionization tendency than aluminum. Therefore, the surface of thepre-aluminum plating film can be replaced with the metal having a lowerionization tendency than aluminum by immersing the pre-aluminum platingfilm together with the substrate in the replacement solution.

When the pre-aluminum plating film is taken out from a first electrolyteafter the first electrolytic treatment step, an oxide film is formed onthe surface of the pre-aluminum plating film by oxygen that is presentin a very small amount in the atmosphere. In the replacement step,aluminum on the surface of the pre-aluminum plating film is dissolvedinto the replacement solution together with the oxide film, and instead,the metal contained in the replacement solution is deposited on thesurface of the pre-aluminum plating film to form a replacement platingfilm. Specifically, aluminum (including the oxide film) on the surfaceof the pre-aluminum plating film is replaced with the metal contained inthe replacement solution, and consequently, a layer made of the metalcontained in the replacement solution is formed on the surface of thepre-aluminum plating film. The layer made of the metal contained in thereplacement solution functions as the intervening layer in the aluminumplating film according to an embodiment of the present disclosure.

The metal contained in the replacement solution does not form a denseoxide film compared with aluminum. Therefore, aluminum electrodepositedin the second electrolytic treatment step, which is performedsubsequently to the replacement step, has high adhesion to a layer madeof a metal contained in a second electrolyte.

The time during which the replacement step is performed is notparticularly limited, and the replacement step is performed so that theoxide film on the surface of the pre-aluminum plating film issufficiently removed. From the viewpoint of making the thickness of theintervening layer as small as possible, it is preferable not to performthe replacement step for a time longer than necessary. The replacementstep is performed in a range of about one second or more and about onehour or less depending on the metal contained in the replacementsolution. In the case where it takes a time to replace aluminum with themetal contained in the replacement solution, the time can be reduced bysupplying a current. The temperature of the replacement solution duringthe replacement step is about 20° C. or higher and about 200° C. orlower.

In the replacement step, the surface of the pre-aluminum plating film isuniformly replaced with the metal contained in the replacement solution.Accordingly, the layer made of the metal contained in the replacementsolution is formed substantially parallel to the surface of thesubstrate (coating surface of the pre-aluminum plating film) as long asthe pre-aluminum plating film is uniformly formed on the surface of thesubstrate in the first electrolytic treatment step.

Replacement Solution

The replacement solution is a solution in which a metal having a lowerionization tendency than aluminum is added to the first electrolyte usedin the first electrolytic treatment step. As the metal having a lowerionization tendency than aluminum, at least one metal selected from thegroup consisting of iron (Fe), zinc (Zn), zirconium (Zr), manganese(Mn), nickel (Ni), and copper (Cu) can be preferably used.

The replacement solution can be easily prepared by dissolving a chlorideof a metal having a lower ionization tendency than aluminum in the firstelectrolyte used in the first electrolytic treatment step. For example,FeCl₄, ZnCl₂, ZrCl₄, MnCl₂, NiCl₂, or CuCl₂ can be used as the chlorideof a metal having a lower ionization tendency than aluminum. Theconcentration of the chloride of a metal having a lower ionizationtendency than aluminum in the replacement solution may be about 10mmol/L or more and about 1 mol/L or less.

Second Electrolytic Treatment Step

The second electrolytic treatment step is a step of forming an aluminumplating film by subjecting the replacement plating film after thereplacement step to an electrolytic treatment in a second electrolyte toelectrodeposit aluminum on a surface of the replacement plating film.

In the related art, in the formation of an aluminum plating film inmultiple stages, adhesion between aluminum plating films formed in therespective stages cannot be increased due to the influence of an oxidefilm that is formed on the surface of an aluminum plating film when thealuminum plating film is pulled up from an electrolyte. In contrast, inthe method for producing an aluminum plating film according to anembodiment of the present disclosure, a dense oxide film of aluminum isnot formed on the surface of the replacement plating film used in thesecond electrolytic treatment step, and thus adhesion between aluminumplating films formed in the respective stages can be increased.

The second electrolytic treatment step can be performed under the sameconditions as those in the first electrolytic treatment step.Specifically, the second electrolytic treatment step can be performed asin the first electrolytic treatment step except that, in the firstelectrolytic treatment step, the replacement plating film after thereplacement step is used instead of using the substrate. It is notessential that the second electrolyte used in the second electrolytictreatment step have the same composition as the first electrolyte usedin the first electrolytic treatment step. However, from the viewpoint ofpreventing another component from entering, electrolytes having the samecomposition are preferably used in the first electrolytic treatment stepand the second electrolytic treatment step.

When the first electrolyte and the second electrolyte have the samecomposition, it is preferable to separately prepare the respectiveplating baths. However, from the viewpoint of space-saving, a platingbath that has been used in the first electrolytic treatment step can beused again in the second electrolytic treatment step. When aluminum isuniformly electrodeposited on the surface of the replacement platingfilm in the second electrolytic treatment step, the layer made of themetal contained in the replacement solution and the surface of thealuminum plating film (coating surface) are substantially parallel toeach other.

Removal Step

An aluminum plating film according to an embodiment of the presentdisclosure can be formed on a surface of a substrate through the firstelectrolytic treatment step, the replacement step, and the secondelectrolytic treatment step. However, in the case where the substrate isunnecessary, the removal step of removing the substrate may beperformed.

The method for removing the substrate is not particularly limited. Forexample, when the substrate is a flat-plate substrate, the substrate canbe removed by detaching the aluminum plating film from the substrate.When the substrate is a resin molded body, the substrate can be removedby, for example, heat treatment. In the case where the resin molded bodyis removed by heat treatment, for example, the aluminum plating film isheated to about 400° C. or higher and about 680° C. or lower in an airatmosphere. In the case where heat treatment is performed, an aluminumplating film in which the metal in the intervening layer is alloyed withaluminum is produced.

EXAMPLES

While the present disclosure will be described in more detail below onthe basis of Examples, these Examples are illustrative, and an aluminumplating film of the present disclosure and a method for producing thealuminum plating film are not limited thereto. The scope of the presentdisclosure is defined by the appended claims and covers allmodifications within the meaning and scope equivalent to those of theclaims.

Example 1 First Electrolytic Treatment Step Substrate

A flat plate-like copper plate (50 mm×80 mm×1 mm) was prepared as asubstrate.

Electrolyte

An electrolyte was prepared as follows. Aluminum chloride (AlCl₃) and1-ethyl-3-methylimidazolium chloride (EMIC) were mixed in a mixing ratioof 2:1 in terms of molar ratio to prepare a molten salt, and1,10-phenanthroline chloride monohydrate serving as a smoothing agentwas added to the molten salt to have a concentration of 0.5 g/L.

Molten Salt Electrolysis

Molten salt electrolysis was performed in the electrolyte prepared asdescribed above in such a manner that the copper plate was used as acathode and an aluminum plate having a purity of 99.99% was used as ananode. As a result, aluminum was electrodeposited on the surface of thecopper plate to form a pre-aluminum plating film. The temperature of theelectrolyte was 45° C. The current density was adjusted to 3.0 A/dm².

Replacement Step Replacement Solution

A replacement solution was prepared by adding FeCl₄ to the electrolyteprepared in the first electrolytic treatment step to have aconcentration of 20 mmol/L.

Replacement Conditions

The copper plate on which the pre-aluminum plating film was formed wasimmersed in the replacement solution prepared as described above. Theimmersion time was 10 seconds, and the temperature of the replacementsolution was 40° C. As a result, a replacement plating film in which thesurface of the pre-aluminum plating film was replaced with Fe wasobtained.

Second Electrolytic Treatment Step

A second electrolytic treatment step was performed as in the firstelectrolytic treatment step except that, in the first electrolytictreatment step, the copper plate on which the replacement plating filmwas formed was used instead of the copper plate.

As a result, an aluminum plating film No. 1 having an intervening layermade of Fe between coating surfaces at both ends in the thicknessdirection was produced. The aluminum plating film No. 1 had a thicknessof about 15 μm.

Example 2

An aluminum plating film No. 2 was produced as in Example 1 except that,in Example 1, a step of removing the copper plate serving as a substrateby immersing the resulting aluminum plating film No. 2 in nitric acidwas added after the second electrolytic treatment step. The aluminumplating film No. 2 had a thickness of about 17 μm.

Example 3 First Electrolytic Treatment Step Substrate

A resin molded body that had been subjected to an electrical conductiontreatment was prepared as a substrate.

The resin molded body used was a polyurethane sheet (50 mm×80 mm) havinga thickness of 1.0 mm and having a skeleton with a three-dimensionalnetwork structure. The resin molded body had a porosity of 96% and anaverage pore diameter of 450 μm.

The electrical conduction treatment was performed by immersing thepolyurethane sheet in a carbon suspension and drying the resultingpolyurethane sheet to form a conductive layer on the surface of theskeleton of the polyurethane sheet. The carbon suspension contained, ascomponents, graphite and carbon black in an amount of 25%, a resinbinder, a penetrant, and an antifoaming agent. The carbon black had aparticle diameter of 0.5 μm.

Electrolyte

An electrolyte that was the same as the electrolyte used in the firstelectrolytic treatment step in Example 1 was prepared as an electrolyte.

Molten Salt Electrolysis

Molten salt electrolysis was performed in the electrolyte prepared asdescribed above in such a manner that the polyurethane sheet that hadbeen subjected to the electrical conduction treatment was used as acathode and an aluminum plate having a purity of 99.99% was used as ananode. As a result, aluminum was electrodeposited on the surface of thepolyurethane sheet that had been subjected to the electrical conductiontreatment to form a pre-aluminum plating film. The temperature of theelectrolyte was 45° C. The current density was adjusted to 6.0 A/dm².

Replacement Step

A replacement step was conducted under the same conditions as those inExample 1. As a result, a replacement plating film was formed on thesurface of the skeleton of the polyurethane sheet.

Second Electrolytic Treatment Step

A second electrolytic treatment step was performed as in the firstelectrolytic treatment step except that, in the first electrolytictreatment step, the polyurethane sheet on which the replacement platingfilm was formed was used instead of the polyurethane sheet that had beensubjected to the electrical conduction treatment.

As a result, an aluminum plating film having an intervening layer madeof Fe between coating surfaces at both ends in the thickness directionwas formed.

Removal Step

The aluminum plating film formed as described above was heated to 600°C. in an air atmosphere to thereby remove the polyurethane sheet bycombustion.

As a result, an aluminum plating film No. 3 that formed a skeleton of ametal porous body having a skeleton with a three-dimensional networkstructure was produced. The aluminum plating film No. 3 had a thicknessof about 15 μm.

Example 4

An aluminum plating film No. 4 was produced as in Example 3 except that,in Example 3, a replacement solution described below was used. Thealuminum plating film No. 4 had a thickness of about 18 μm.

Replacement Solution

The replacement solution was prepared by adding ZnCl₂ to the electrolyteused in the first electrolytic treatment step in Example 1 to have aconcentration of 40 mmol/L.

Example 5

An aluminum plating film No. 5 was produced as in Example 3 except that,in Example 3, a replacement solution described below was used. Thealuminum plating film No. 5 had a thickness of about 16 μm.

Replacement Solution

The replacement solution was prepared by adding ZrCl₄ to the electrolyteused in the first electrolytic treatment step in Example 1 to have aconcentration of 100 mmol/L.

Comparative Example 1

An aluminum plating film No. 6 was produced as in Example 2 except that,in Example 2, the second electrolytic treatment step was performedwithout performing the replacement step. The aluminum plating film No. 6had a thickness of about 16 μm.

Comparative Example 2

An aluminum plating film No. 7 was produced as in Example 3 except that,in Example 3, the second electrolytic treatment step was performedwithout performing the replacement step. The aluminum plating film No. 7had a thickness of about 18 μm.

Evaluation

The aluminum plating film Nos. 1 to 7 produced as described above wereevaluated as described below. Table 1 shows the evaluation results.

Measurement of Conductivity

The resistivity of each of the aluminum plating film Nos. 2 to 7 wasmeasured with a resistivity meter so that the distance between measuringterminals was 60 mm.

Table 1 shows the results.

TABLE 1 Thickness of Aluminum plating film aluminum plating filmResistivity No. (μm) (μΩ · m) 1 15 — 2 17 0.038 3 15 1.82 4 18 1.78 5 161.88 6 16 0.062 7 18 2.09

As shown in Table 1, comparing the aluminum plating film No. 2 ofExample 2 and the aluminum plating film No. 6 of Comparative Example 1,which were produced by the method for forming an aluminum plating filmon a surface of a copper plate, the aluminum plating film No. 2 had alower resistivity. An aluminum film produced by rolling and having athickness of 18 μm had a resistivity of 0.031 μΩ·m. This showed that thealuminum plating film No. 2 of Example 2 had a low resistivity that issubstantially comparable to the resistivity of the film produced byrolling.

Comparing the aluminum plating film Nos. 3 to 5 of Examples 3 to 5 andthe aluminum plating film No. 7 of Comparative Example 2, which wereproduced by the method for forming an aluminum plating film on a surfaceof the skeleton of a polyurethane sheet, the aluminum plating film Nos.3 to 5 each had a lower resistivity.

Observation of Section of Aluminum Plating Film

A section of each of the aluminum plating film Nos. 3, 4, and 7 wasobserved with a scanning electron microscope (SEM). FIGS. 7 to 9 showthe results.

As shown in FIGS. 7 and 8, the aluminum plating film Nos. 3 and 4according to embodiments of the present disclosure had interveninglayers 72 and 82, respectively, between coating surfaces at both ends inthe thickness direction. In addition, the aluminum layers on both sidesof the intervening layer were in close contact with each other without agap therebetween.

In contrast, as illustrated in FIG. 9, the aluminum plating film No. 7of Comparative Example 2, in which the replacement step was notperformed, had a gap between the aluminum plating film formed in thefirst electrolytic treatment step and the aluminum plating film formedin the second electrolytic treatment step and had poor adhesion.Furthermore, the aluminum plating film formed in the second electrolytictreatment step was partially detached.

REFERENCE SIGNS LIST

10 aluminum plating film

11 coating surface

11′ coating surface

12 intervening layer

20 aluminum plating film

21 coating surface

21′ coating surface

22 intervening layer

30 aluminum plating film

31 coating surface

31′ coating surface

32 intervening layer

33 skeleton

34 inside of skeleton

35 pore

65 pore

66 resin molded body

67 conductive layer

71 coating surface

71′ coating surface

72 intervening layer

81 coating surface

81′ coating surface

82 intervening layer

1. An aluminum plating film comprising aluminum as a main component,wherein the aluminum plating film has, between coating surfaces at bothends in a thickness direction, an intervening layer that contains ametal having a lower ionization tendency than aluminum or an interveninglayer that contains an alloy of aluminum and a metal having a lowerionization tendency than aluminum.
 2. The aluminum plating filmaccording to claim 1, wherein the aluminum plating film has an elongatedsheet-like shape.
 3. The aluminum plating film according to claim 1,wherein the aluminum plating film forms a skeleton of a metal porousbody, the skeleton having a three-dimensional network structure.
 4. Thealuminum plating film according to claim 3, wherein the metal porousbody has an elongated sheet-like shape.
 5. The aluminum plating filmaccording to claim 1, wherein the aluminum plating film has a pluralityof the intervening layers in the thickness direction of the aluminumplating film.
 6. The aluminum plating film according to claim 1, whereinthe metal having a lower ionization tendency than aluminum is at leastone selected from the group consisting of iron, zinc, zirconium,manganese, nickel, and copper.
 7. The aluminum plating film according toclaim 1, wherein the aluminum plating film has a thickness of 10 μm ormore and 1,000 μm or less.
 8. A method for producing the aluminumplating film according to claim 1, the method comprising: a firstelectrolytic treatment step of forming a pre-aluminum plating film bysubjecting a substrate, at least a surface of the substrate beingconductive, to an electrolytic treatment in a first electrolyte toelectrodeposit aluminum on a surface of the substrate; a replacementstep of, after the first electrolytic treatment step, forming areplacement plating film by immersing the pre-aluminum plating film in areplacement solution that contains the first electrolyte and a metalhaving a lower ionization tendency than aluminum to replace a surface ofthe pre-aluminum plating film with the metal having a lower ionizationtendency than aluminum; and a second electrolytic treatment step of,after the replacement step, forming an aluminum plating film bysubjecting the replacement plating film to an electrolytic treatment ina second electrolyte to electrodeposit aluminum on a surface of thereplacement plating film, wherein the first electrolyte and the secondelectrolyte are each a molten salt that contains at least aluminumchloride.
 9. The method for producing the aluminum plating filmaccording to claim 8, wherein the substrate is a resin molded bodyhaving a skeleton with a three-dimensional network structure.
 10. Themethod for producing the aluminum plating film according to claim 8,comprising a removal step of removing the substrate after the secondelectrolytic treatment step.